Investigation of superconductivity in the single-walled carbon nanotubes

Superconductivity in the single-walled carbon nanotubes is investigated. First, effect of diameter increasing on the clean systems critical temperature, T_c, is calculated. Then effect of impurity doping on the reduction of critical temperature T_c, of single-walled carbon nanotubes, is discussed. Our calculations illustrate that metallic zigzag single-walled carbon nanotubes have higher T_c than armchair single-walled carbon nanotubes with approximately same diameters and T_c decreases by increasing diameter. This can explain why superconductivity could be found in the small diameter single-walled carbon nanotubes. We found for the impurity doped systems, impurity in the strong scattering regime can decrease T_c significantly while in the weak scattering regime T_c is not affected by impurity doping.     

Electron-phonon induced superconductivity in highT c superconductors

It is proposed that phonon induced mechanism is capable of explaining the observed range ofT _c for the newly discovered high temperature superconductor within the Allen and Dynes strong coupling theory.     

Photo-induced superconductivity

Recent advances in laser technology have made it possible to generate of precisely shaped strong-field pulses at terahertz frequencies. These pulses are especially useful to selectively drive collective modes of solids, for example, to drive materials in a fashion similar to what done in the synthetic environment of optical lattices. One of the most interesting applications involves the creation of non-equilibrium phases with new emergent properties. Here, I discuss coherent control of the lattice to favour superconductivity at ‘ultra-high’ temperatures, sometimes far above the thermodynamic critical temperature T_c.     

Fractal superconductivity near localization threshold

We develop a semi-quantitative theory of electron pairing and resulting superconductivity in bulk “poor conductors” in which Fermi energy E_F is located in the region of localized states not so far from the Anderson mobility edge E_c. We assume attractive interaction between electrons near the Fermi surface. We review the existing theories and experimental data and argue that a large class of disordered films is described by this model.Our theoretical analysis is based on analytical treatment of pairing correlations, described in the basis of the exact single-particle eigenstates of the 3D Anderson model, which we combine with numerical data on eigenfunction correlations. Fractal nature of critical wavefunction's correlations is shown to be crucial for the physics of these systems.We identify three distinct phases: ‘critical’ superconductive state formed at E_F = E_c, superconducting state with a strong pseudo-gap, realized due to pairing of weakly localized electrons and insulating state realized at E_F still deeper inside a localized band. The ‘critical’ superconducting phase is characterized by the enhancement of the transition temperature with respect to BCS result, by the inhomogeneous spatial distribution of superconductive order parameter and local density of states. The major new feature of the pseudo-gapped state is the presence of two independent energy scales: superconducting gap Δ, that is due to many-body correlations and a new “pseudo-gap” energy scale Δ_P which characterizes typical binding energy of localized electron pairs and leads to the insulating behavior of the resistivity as a function of temperature above superconductive T_c. Two gap nature of the pseudo-gapped superconductor is shown to lead to specific features seen in scanning tunneling spectroscopy and point-contact Andreev spectroscopy. We predict that pseudo-gapped superconducting state demonstrates anomalous behavior of the optical spectral weight. The insulating state is realized due to the presence of local pairing gap but without superconducting correlations; it is characterized by a hard insulating gap in the density of single electrons and by purely activated low-temperature resistivity ln R(T) ∼ 1/T.Based on these results we propose a new “pseudo-spin” scenario of superconductor-insulator transition and argue that it is realized in a particular class of disordered superconducting films. We conclude by the discussion of the experimental predictions of the theory and the theoretical issues that remain unsolved.     

Odd-frequency superconductivity on quasi-one-dimensional triangular lattice in the Hubbard model

We study the superconductivity in the Hubbard model on quasi-one-dimensional triangular lattice using random phase approximation (RPA). We find that odd-frequency spin-singlet p-wave pairing can be realized on isosceles quasi-one-dimensional triangular lattice.     

Influence of disorder on superconductivity in non-magnetic rare-earth nickel borocarbides

The effect of substitutional disorder on the superconducting properties of YNi₂B₂C was studied by partially replacing yttrium and nickel by Lu and Pt, respectively. For the two series of (Y, Lu)Ni₂B₂C and Y(Ni, Pt)₂B₂C compounds, the upper critical field H _c2(T) and the specific heat c _p(T, H) in the superconducting mixed state have been investigated. Disorder is found to reduce several relevant quantities such as T _c, the upper critical field H _c2(0) at T=0 and a characteristic positive curvature of H _c2(T) observed for these compounds near T _c. The H _c2(T) data point to the clean limit for (Y, Lu) substitutions and to a transition to the quasi-dirty limit for (Ni, Pt) substitutions. The electronic specific heat contribution γ(H) exhibits significant deviations from the usual linear γ(H) law. These deviations reduce with growing substitutional disorder but remain even in the quasidirty limit which is reached in the Y(Ni_1−x , Pt _x )₂B₂C samples for x=0.1.     

Superfluidity and superconductivity in the universe

The paper aims to elucidate the current status of the problem concerning the existence and observation of superfluid and superconducting states in the universe, that is, under cosmic conditions. Following an introduction, the paper discusses Bose-Einstein condensation, superfluidity, and superconductivity; possibilities for the occurrence of superfluidity and superconductivity under cosmic conditions; superconductivity of dense, degenerate electron plasma (large planets, white dwarfs); superfluidity and superconductivity in neutron stars; and finally superfluidity in a cosmological neutrino “sea.”     

两带模型与MgB2的超导电性

文中基于两带模型,通过引入非电-声短程相互作用,在弱耦合极限下按实验结果选定一组参数,同时得到了MgB2超导体的临界温度、同位素效应指数、零温能隙及比热的跃变,这些结果与实验相符合。     

Evolution of metallization and superconductivity in solid hydrogen

Inspired by the recent experimental reports of metallic hydrogen [Science 355 (2017) 715, Nature, 577 (2020) 631], we have reexamined the metallization and superconductivity of solid hydrogen in the pressure range of interest. Based on high quality calculations with zero-point vibrations and van der Waals (vdW) corrections, hydrogen is disclosed to metallize at about 485 GPa via the phase transition from insulate molecular C2/c-24 to metallic molecular Cmca-4, then dissociate into atomic phase with increasing pressure to 600 GPa. Further analyses demonstrate that vdW interaction can reduce the H-H distances in the metallic molecular Cmca-4, thus pulling down the contributions from phonon frequencies and electronic structures to electron-phonon coupling λ, and resulting in the declining superconducting $T_{\mathrm{c}}$. Meanwhile, slightly influence on these cases can be found in metallic atomic I4₁/amd by vdW correction. Our results indicate that aspirational room-temperature superconductivity in solid hydrogen requires a high pressure beyond 600 GPa.     

Possible superconductivity in metallic Xenon

Abstract

We report here metallization and possible superconductivity in Xenon when it is subjected to a pressure of the order of 137 GPa. The metal is found to be in the hcp phase at this pressure and our band structure calculations done using the linear muffin-tin orbital method show that the band gap closes around this pressure.     

D-wave superconductivity in doped Mott insulators

The effect of proximity to a Mott insulating phase on the charge transport properties of a superconductor is determined. An action describing the low energy physics is formulated and different scenarios for the approach to the Mott phase are distinguished by different variation with doping of the parameters in the action. A crucial issue is found to be the doping dependence of the quasiparticle charge which is defined here and which controls the temperature and field dependence of the electromagnetic response functions. Presently available data on high-T_c superconductors are analyzed. The data, while neither complete nor entirely consistent, suggest that neither the quasiparticle velocity nor the quasiparticle charge vanish as the Mott phase is approached, in contradiction to the predictions of several widely studied theories of lightly doped Mott insulators. Implications of the results for the structure of vortices in high-T_c superconductors are determined.     

Criterion for superconductivity in highT c oxidic compounds

A criterion for superconductivity in highT _c oxidic compounds is proposed based on a short range cooperative electron-phonon coupling and the Darwin interaction for Cooper pairing. It is shown that the small polaron stabilization energy (ε _p) should be less than about 0.04 eV and the system should have a metal ion for which Darwin interaction energy is large.     

La_(1．6-x)Nd_(0．4)Sr_xCuO_4超导电性的反常特性

分析并讨论了La_(1．6-x)Nd_(0．4)Sr_x CuO_4系列单相性样品(x=0．08～0．25)的晶体结构和超导电性。结果表明,晶体结构随着Sr掺杂量的变化规律与La_(2-x)Sr_x CuO_4系列相似。超导电性方面,La_(1．6-x)Nd_(0．4)Sr_x CuO_4系列超导转变温度均远小于相应的La_(2-x)Sr_xCuO_4系列组分。同La_(1．875)Ba_(0．125)CuO_4相似,La_(1．48)Nd_(0．4)Sr_(0．12)CuO_4处出现显著的极小值8K但高于La_(1．875)Ba_(0．125)CuO_4。La_(1．6-x)Nd_(0．4)Sr_x CuO_4系列的超导电性明显与La_(2-x)Sr_x CuO_4系列有着不同的特性,虽然同La_(2-x) Ba_xCuO_4系列相似但仍有区别。我们从其晶体结构的反常变化的角度对La_(1．6-x)Nd_(0．4)Sr_x CuO_4系列反常的超导电性做了初步的解释。     

晶格常数与掺杂YBCO体系超导电性的关系

依据现有的YBCO掺杂实验,对掺杂YBCO体系超导电性与其晶格常数进行了研究,发现它们之间具有较好的规律性。因此提出用晶格常数作为掺杂YBCO超导电性的一个判断标准,这对探索超导机制及对今后实验工作掺杂元素和掺杂比例的选取有很好的指导意义。     

A precursor superconductivity approach to magnetic field effects in the pseudogap phase

We demonstrate that the observed dependences of T_c and T^∗ on small magnetic fields can be readily understood in a precursor superconductivity approach to the pseudogap phase. In this approach, the presence of a pseudogap at T_c (but not at T^∗) and the associated suppression of the density of states lead to very different sensitivities to pair-breaking perturbations for the two temperatures. Our semi-quantitative results address the puzzling experimental observation that the coherence length ξ is weakly dependent on hole concentration x throughout most of the phase diagram. We present our results in a form which can be compared with the recent experiments of Shibauchi et al. and argue that orbital effects contribute in an important way to the H dependence of T^∗.     

Electronic structure, magnetism and superconductivity of layered iron compounds

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe (Se, Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.     

Solitary re-entrant superconductivity in asymmetrical FSF structures

Solving the boundary value problem for the Eilenberger function, the superconducting and magnetic states of asymmetric ferromagnet–superconductor–ferromagnet (F₁SF₂) nanostructures are investigated. The dependences of critical temperature on an exchange field of the F metal, electronic correlations in the S and F metals, and thicknesses of layers F and S are derived. It is shown that the possibility of the Larkin–Ovchinnikov–Fulde–Ferrell (LOFF) state observation is especially increased in the asymmetrical trilayers F₁SF₂ for which solitary re-entrant superconductivity is predicted. The possibility of solitary re-entrant superconductivity for asymmetrical trilayers F₁SF₂ in the dirty limit is also shown.